of from four to five kilograms
weight, and containing from fifty to sixty per cent of copper.

[The copper "stone".] Several of these "stones" were melted down
together for the space of about fifteen hours, in a powerful fire;
and by this means a great portion of the three volatile substances
above named was again evolved; after which they placed them, now heated
red-hot, in an upright position, but so as to be in contact with the
draught; the coals, however, being at the sides of the furnace. After
blowing for an hour or half-an-hour, they thus obtained, as residuum,
a silicate of iron with antimony and traces of arsenic, a "stone"
containing from seventy to seventy-five per cent of copper, which they
took off in very thin strips, at the same time using refrigerating
vessels; and at the bottom of the hollow there remained, according
as the mass was more or less freed from sulphur, a larger or smaller
quantity (always, however, impure) of black copper.

[Purifying the product.] The purified stones obtained by this second
process were again made red-hot by placing them between rows of wood,
in order that they might not melt into one another before the fire
had freed them from impurities.

The black copper obtained from the second operation, and the stones
which were re-melted at the same time, were then subjected to a
third process in the same furnace (narrowed by quarry stones and
provided with a crucible); which produced a residuum of silicious
iron and black copper, which was poured out into clay moulds, and
in this shape came into commerce. This black copper contained from
ninety-two to ninety-four per cent of copper, and was tinged by a
carbonaceous compound of the same metal known by its yellow color,
and the oxide on the surface arising from the slow cooling, which will
occur notwithstanding every precaution; and the surface so exposed
to oxidation they beat with green twigs. When the copper, which had
been thus extracted with so much skill and patience by the Igorots,
was to be employed in the manufacture of kettles, pipes, and other
domestic articles, or for ornament, it was submitted to another
process of purification, which differed from the preceding only in
one particular, that the quantity of coals was diminished and the
air-draught increased according as the process of smelting drew near
to its termination, which involved the removal of the carbonaceous
compound by oxidation. Santos found, by repeated